Conventional serial bit-oriented information storage devices are approaching their physical storage capacity limit. Multidimensional storage devices having novel recording mechanisms, redundant storage, and parallel recording and retrieval potentials may be able to display higher storage capacity and transfer rates. In addition, when one recorded symbol is able to represent many bits rather than one bit, multilevel data storage may be able to push the storage capacity and data transfer rate even higher.
One such storage device records information as multiplexed interference patterns in different common volumes within holographic media, from which the patterns can be independently retrieved. Page-oriented recording and retrieval improves the data transfer rate.
In holographic storage a single monochromatic, coherent beam of light is preprocessed and passed through a spatial light modulator which is controlled by a sequence of digital data arrays. The modulator is in the form of a 2-Dimensional array of pixels each of which has a particular level of brightness. The level of brightness, q, at a given pixel location represents log2 q bits. The modulated, light beam exiting the spatial light modulator carries the information of the multilevel data array as multilevel symbols.
In Fourier hologram recording geometry, the signal light is the Fourier transform of the light exiting the spatial light modulator. This Fourier transform represents the spatial frequency content of the multilevel data array on the spatial light modulator. A lens is interposed between the spatial light modulator and the recording medium to obtain the Fourier transform of the light.
This resulting signal beam is interfered with a coherent reference beam at or near the recording plane. The resulting interference pattern has bright bands where the signal and the reference wavefronts constructively interfere with and enhance one another, and dark bands where the wavefronts destructively interfere and cancel each other out.
The interference pattern is then recorded as a 3-Dimensional hologram in a holographic recording medium, as variations of the index of refraction of the medium. The pattern of the resultant refraction index change has a half period shift with respect to the interference pattern.
Since the holographic medium has finite dynamic range of index of refraction, the brighter the interference pattern is, the greater the refraction index will change, and the less index change range will be left. If the input multilevel symbols are not distributed evenly in the data array, the interference pattern will have peaks which will saturate the recording medium. In this event the interference process acts like a nonlinear filter which makes the recording and reconstruction not a true reproduction of the original input multilevel data array.
The multilevel image pages are reconstructed by illuminating the recorded hologram with the original reference beam and detecting the readout with a matched 2-Dimensional array of photo-detectors, such as charge coupled devices. The multilevel data array can then be retrieved from the detected image, and the original data array can be decoded from the data array.
Holographic storage records the modulation depth of the interference between the reference and the signal beams. The modulation depth is determined by the relative ratio between the two beams. Therefore, wide variations in the ratio between signal beam and the reference beam during the recording will result in wide variations in the intensity of the reconstructed image.